Power over Ethernet (PoE) is a technology for supplying low voltage current and data over a common point-to-point Ethernet network cable to locations with applications that require both mediums. In some cases, power is carried on the same conductors that carry data. In other cases, power is carried on dedicated conductors within the same cable as the conductors that carry data. Applications that currently benefit from PoE technology include Voice over Internet Protocol (VoIP), Internet Protocol (IP) cameras, wireless local area networks (WLAN), Wireless Access Points (WAP), Building Automation Systems (BAS), and security and access control systems.
PoE currently has two standards: Institute of Electrical and Electronics Engineers (IEEE) 802.3 of (the original PoE standard) and IEEE 802.3 at (known as PoE plus), which provide, respectively, about 13 Watts and about 25.5 Watts of power to connected devices. PoE has several advantages over traditional power systems used in homes and commercial buildings. For example, PoE systems are relatively low voltage, thus eliminating the need to run expensive high voltage wiring and conduit for lighting. In addition, installation of PoE wiring can be faster than with traditional power systems because Ethernet cabling employs simple plug-in end connections. Where Ethernet cabling is already in place (i.e., for data transmission), PoE functionality can be achieved without the need for additional wiring installation.
Light emitting diode (LED) luminaires can benefit from connection to a PoE network. Recent advances have reduced the power required to operate LED luminaires to a point where network switches that are compliant with PoE standards, such as IEEE 802.3 at, can supply the power required by the LED luminaires. In addition, digital Ethernet communications can be used to command the LED luminaires to dim and brighten, as well as to report status such as lamp failure and energy consumption.
With higher power PoE systems, such as standard IEEE 802.3 at (25.5 Watts), and especially with proposed standards such as IEEE 802.3 bt, which may allow 60-100 Watts of power to be supplied, it is possible that excessive heating can occur if the category of cable used to connect the power sourcing equipment (e.g., PoE switches, PoE midspan devices) to the powered devices such as LED drivers, is of insufficient gauge to accommodate the power level.
This problem may be exacerbated by PoE LED Drivers because many PoE LED drivers deliver constant power (current or voltage) to connected LED light fixtures, regardless of input voltage, due to the nature of the switching circuit commonly employed to drive LED light sources. In such cases, as the cable heats up due to current flow, the resistance of the cable will increase, thus causing the voltage drop across the cable to increase and the input voltage to the LED driver to drop. In response, the LED driver may increase input current to maintain constant power to the LED light fixture which, in turn, can cause additional cable heating, further lowering the driver input voltage and further increasing the current supplied to the fixture. As will be appreciated, this can cause a thermal-runaway condition with respect to the cable, and may continue until the system fails.
It would, therefore be desirable to provide a PoE LED lighting system that is capable of sensing when a cable is approaching an excessive heating condition and can automatically change the operating characteristics of the PoE LED driver to ensure that overheating and system failure do not occur. Such a system should also provide information and/or an alert to the system or user to ensure that corrective action is taken.